Magnetic Fe₃O₄@ polystyrene (PS) nanoscale composite particles were fabricated using a unique Shirasu porous-glass membrane technique and used as additives to a magnetorheological (MR) suspension with magnetic carbonyl-iron (CI) particles. The morphology of the fabricated Fe₃O₄@ PS composites was characterized by transmission electron microscopy. The effects of the Fe₃O₄@ PS composite particle additive on its MR behavior was examined using a rotation rheometer under an applied magnetic field. The results revealed improved MR behavior, such as higher storage and loss moduli, and elastic yield stress compared to those of the pure CIbased MR suspension under an external magnetic field.